The 32-year-old Scot, who is aiming for a possible three Olympic gold medals in Beijing, also handed Britain their first title in the men's event since Reginald Harris's win in the professional category and Cyril Peacock's victory in the amateur ranks, both in 1954.
"I never dreamt that I'd win this," Hoy said.
"I came here with an outside chance of a medal. I had a couple of good rides in the World Cup, but my form just seems to be stepping up and up.
"It's the most surprising, even more so than the keirin last year."
|"I had a couple of good rides in the World Cup, but my form just seems to be stepping up and up."|
Hoy's victory was all the more deserved as he ousted defending champion Theo Bos in the quarter-finals before going on to beat Sireau, who had dominated qualifying with the only sub 10-second time of the field.
The new star of French sprinting, who is only 20 years old, was disappointed at not picking up his second gold of the event.
"It's my first world championship medal (in individual). But I'm totally gutted," said Sireau, who claimed gold on the opening day as France dominated Britain in the team sprint.
"I had total belief that I could win both legs, and gave it everything I had."
Romero doubles up
Britain had already celebrated their fifth gold of the championships when Rebecca Romero, who won individual pursuit gold on Thursday, teamed up with Wendy Houvenaghel and Joanna Rowsell to crush Ukraine in the team pursuit final.
Although the women's team pursuit is not an Olympic event, Romero will be Britain's best hope of individual pursuit gold in Beijing, where she will also aim for the rare feat of winning Olympic medals from two sports.
"Yesterday was phenomenal. At two o'clock in the morning I was still awake with a smile on my face," Romero, a former rower who won silver in the quadruple sculls in Athens, told AFP.
"But it was a case of getting the head down knowing you had another job to do.
"It was quite tough, but being so elated from yesterday and running on adrenaline helped me through."
Prior to the feats of Hoy and the women's pursuit trio, Britain had already won four gold medals from seven events, through Bradley Wiggins in the individual pursuit, Victoria Pendleton and Shanaze Reade in the team sprint, the men's pursuit team and Romero in the women's individual pursuit.